Shards for cores

In your opinion, is it better to run the 50 dungeons needed to get 2000 shards or run the defiler? I know he drops more, but I don't quite have a build ready for him yet but I'm getting consistent on at least every other battle of winning. Wish I could do it every time but I'm working on it. Ran out of catalyst, so now playing the waiting game. But I know I can run 50 dungeons for the same amount. Is there a benefit to running defiler over dungeons?